This reverse-circulation drilling technique can be considered as non-conventional in relation to standard drilling with direct circulation through the inside of the rods wherein the surface injection pressure often ranges between 100 and 300 bars, considering mainly the pressure drops in the string, and the pressure dissipated by the nozzles of the bit for cleaning and cooling it, and for eroding the formation. The cuttings-laden mud flows back up through the annular space between the borehole and the drill string.
Reverse-circulation drilling consists in injecting the drilling fluid at the surface through the annular space and to circulate the cuttings-laden fluid back up through the inside of the drill string.